Power converters are widely used to provide power having the appropriate electrical characteristics to equipment (“loads”). Power converters convert electric power from one form (e.g., relatively high voltage alternating current, or AC) to another (e.g., lower voltage direct current, or DC). Converters may convert from DC to AC (“inverters”), AC to AC (“cycloconverters”), AC to DC (“rectifiers”) or DC to DC (“switch mode power supplies” or “choppers”). Most power converters operate by using magnetic components, such as transformers and inductors, to transfer or filter electric power. Many power converters use solid state switches (transistors) to make and break electrical connections quickly to create multiple paths for electricity through the converter. The multiple paths impart the desired characteristics to the electric power.
Some loads require substantial quantities of electric power and consequently a relatively large power converter. One way to address this need is to provide the converter with larger components (e.g., magnetic components and switches). Unfortunately, larger components have two significant disadvantages. First, they are expensive and harder to obtain in quantity. Second, they are subject to increased variations in operation; magnetic components often vary more widely from one to another in terms of inductance, and switches may vary more from one to another in terms of switching threshold or response time.
Another way to address the need for more output power is to couple multiple smaller power converters in parallel. Each power converter may be thought of and referred to as a “power train.” The outputs of the power trains are simply connected together and to the load. This approach avoids the above-noted problems associated with larger components.
Unfortunately, another problem arises; the multiple power trains must work in unison to provide the needed power. However, if the components of one power train differ in their operation from the components in another, the power trains are unbalanced and may actually supply power to one another in addition to the load. Magnetic components typically fall into two categories: transformers and inductors. Transformers typically vary in inductance enough to cause significant circulating magnetizing currents to flow from one power train to another. Imbalanced magnetizing currents forced through inductors, typically located at the output of a power train, cause voltage spikes of such magnitude to overwhelm the rectifying switches changing their electrical characteristics and even destroying them.
The potentially serious problem of magnetizing current imbalance has been dealt with two different ways. In some designs, particular care has been taken to match the characteristics of the magnetic components such that the magnetizing currents are balanced. Unfortunately, matched components are expensive; many components must be manufactured to yield a suitably matched pair.
Other designs add snubber circuits to reduce the voltage spikes resulting from imbalanced magnetizing currents. However, snubber circuits represent additional circuitry and thus additional cost. This approach also typically results in additional power dissipation. Minimizing cost and power dissipation are goals that pervades power converter design efforts.
Accordingly, what is needed in the art is a way to reduce magnetizing current imbalance in parallel power trains that does not require matched magnetic components. What is further needed in the art is a method of reducing voltage spikes that arise as a result of magnetizing current imbalances that does not require snubber circuits and a power converter that employs such circuit or method.